The present invention is related to a circuit for a system of contactless inductive energy transfer, especially for application in energy supply of mobile devices as well as a related charging circuit.
The principle of inductive energy transfer serves in a plurality of applications as physical basis of technical development of a further field of applications. The principle division of a system for inductive energy transfer is shown in FIG. 1. An essential element in case of a contactless inductive energy transfer is a loosely coupled conductor, which represents magnetic coupling of an inductor in the base part with an inductor in the mobile part on a circuit device base. In operating condition of FIG. 1a, energy transfer between base part and mobile part takes place. This energy can be utilized on the one hand to enable functionality of the mobile part, and on the other hand can be buffered in accumulator batteries (for modern applications mostly Li-ion accumulators, although further types of accumulator batteries like lead-, NiCd-, NiMh-types can be used). If the mobile part 104 is removed from the base part 102 as shown in FIG. 1b, the energy transfer is interrupted. The mobile part 104 is then supplied by the previously charged internal energy storage or remains in inactive state until the next contact with the base part 102.
Due to the special arrangement of the mobile part 104 close to the base part 102, energy transfer between the base part and the mobile part is made possible. The most popular example of such an inductive charging system is the electric toothbrush, which enables charging of the toothbrush as mobile part 104 without galvanic connection.
Omission of galvanic leads is of great importance for manifold usage scenarios. This applies specifically to applications with high demands in the mechanical set up of the electric connections between the power source and sink in which technically complex plugs and cables can be avoided by application of inductive energy transfer (IE). Further, technical energy supply system components based on IE can be protected from environmental impacts without making the mechanical set up unnecessarily complex by appliance of outsourced connectors. Moreover, existing areas of operation for IE in which application of galvanic connections in view of technical feasibility has to be avoided, for example in explosion prone environments or during operation of the system components in conductive and or aggressive media. Furthermore, the use of IE can improve the reliability of technical systems. This is the case on the one hand for systems with rotating or moveable parts since wiper contacts that are prone to attrition can be omitted and on the other hand in devices with connectors which in any other case would have to be dimensioned for a plurality of plugs.
In connection with the increasing penetration of the market by technical solutions based on the principles of IE, the aspect of user friendliness should be emphasized. Especially in the field of portable electrical power sinks, this additional value due to the simplified handling becomes apparent for the user which can initiate supply of these portable devices by only placing the mobile part at the charging station.
The present state of the art mostly includes a signal feedback between power sink and power source in applications of IE due to which the present electric state of the galvanically separated secondary side is known in some form on the primary side. This information is used on the primary side by changing a control value (switching frequency duty cycle etc) on the primary side in order to answer to a change in the load on the secondary side. This technical approach requires provision of a channel for information transfer. Known technical realizations, for example DE 3902618 A1, DE 10158794 B4 and U.S. Pat. No. 6,912,137 B2 use a separate physical channel for this signal feedback as shown in FIG. 2.
From a technical set up point of view, this approach is sophisticated because constructive limitations have to be accepted (alignment of the mobile part in relation to the base part can only take place to a limited extent, in case of an optical path providing the optical components or optical fibers outside respectively utilizing transparent materials is necessary, in case of separate magnetic couplings a second inductor pair is indispensible) and further additional expenditure of components and circuits arises.
Alternative approaches use the same magnetic coupling for the signal feedback as for the energy transfer itself (EP288791 B1, EP 982831 A2). This approach is shown in FIG. 3. This known way of feedback is to be seen as disadvantageous in view of construction of the primary and secondary side windings since these have to be adopted for transmitting signal information as well as for the energy transfer.
A further alternative solution exists in utilizsation of wirelessly supported solutions (for example U.S. Pat. No. 6,436,299 B1) based on the propagation of electromagnetic waves. FIG. 4 depicts this possibility.
This technical solution also constitutes an increased technical constructive effort. The necessary components for provision of the wireless channel on the one hand have to be placed in the parts of the devices especially the antennas have to be placed appropriate to one another. Due to this, limitations during operation occur in view of the alignment of the parts of the devices to one another. Moreover, the telecommunication reconditioning of the radio signal increases the circuit device effort.
In case that a channel for information exchange between the base part and the mobile part is available, different modulation schemes (fm, am, etc) are deployed in the existing technical solutions. This telecommunicational conditioning results in an increased demand in the technical realization of the system independent of the kind of the used channel. This is an additional serious disadvantage of known solutions based on the feedback of a signal.
The principle construction of the power part of a system for IE on the basis of resonant DC DC converters is shown in FIG. 5. This converter type is identified in connection with inductive energy transfer as state of the art. Besides this further converter types on the basis of transformers can be thought of (flyback, forward, CUK, asymmetric half bridge etc). The input voltage Vi is cut up by a switch bridge 106 into a high frequency AC voltage. This switch bridge 106 consists of a half bridge, respectively full bridge, wherein semiconductor switches as active components are deployed. This AC voltage is applied to the primary side of the loosely coupled transformers 110. For compensating the comparably high reactive components of the transformers, further reactive components are provided on the primary side and the secondary side which are schematically depicted as resonant circuits 108 and 112. As a general rule, a serial capacity is integrated in the primary side although further reactive components can be provided for purposefully manipulation of the frequency properties of the primary circuit.
On the secondary side usage of additional reactive components can be omitted (LLC) although further capacities for compensating the main inductions of the conductor can be used (LLCC) in parallel as well as in serial circuits. Moreover, usage of additional reactive components is possible again for purposefully manipulation of the frequency characteristics of the secondary side.
The secondary current is rectified on the output side. The rectification 114 can be carried out as half way rectification or full way rectification, the components can be regular diodes as well as semi-conductor switches (synchronous rectification). The rectified output current is smoothed with the help of the filter 116 (optional with an inductivity). The present state of the art uses a feedback signal in order to realize a comparison of the nominal value and actual value, for example in order to follow up the switching frequency as a control variable of the controlled system in case of a resonant converter.
There are other technical approaches allowing for the omission of the signal feedback in case of a resonant converter by a restricted choice of the bias point. These are based on the determination of the switching frequency to a constant value. In order to limit the variation of the input voltage at different coupling ratios in these approaches it is suggested to carry out the operation at a so called “coupling independent point” a bias point at which under condition that the resistive charging of the source is equal, an approximately equal output voltage is achieved.
Often these determined special bias points are based on simplified calculation methods. If one examines the adjusted output voltage in view of the charging for different spaces for a system for IE in more detail, one obtains the characteristic trend as shown in FIG. 6. Here for each switching frequency of a half bridge the appearing output voltage under workload of the output voltage with the nominal output power of the system is determined. Additionally, the open circuit voltage is shown dashed.
As can be seen from these trends, this characteristic property of a coupling independent point for the shown exemplarily chosen system is indeed comprehensible (choice of switching frequency at fs≈48 kHz for IG=3 mm and IG=6 mm) although in a practical system being available only under significant constraints. Hence it is apparent that for poor coupling conditions in the chosen example for IG=8 mm the property of a preferably constant output voltage is lost. Moreover, a variation of the value of the output voltage occurs when the output is loaded with a capacity different from the nominal capacity especially in case of idle running occurring in almost every application.
By using the inventive concept described in the following embodiments, several problems in view of the technical realization of a system for IE are eliminated. By using a feedback signal the associated signal path has to be realized either separate or has to be integrated with significant influence on the construction of the magnetic coupling in the principle energy current path. Both solutions result in increased technical complexity. By using the concept described herein it can be provided for an optimal mechanical and electrical construction of the total system in view of the energy transfer by omission of the signal feedback.
A further technical problem arises from the exploitation of the coupling independent bias point. On the one hand, an interval comprising an intercept point of the upper voltage for the distances IG=3 mm and IG=6 mm can be recognized from the chosen example of FIG. 6 in an interval between 45 and 50 kHz. However, as soon as the distance is increased, the magnetic coupling decreases such that the coupling independence is lost. This holds in principle for each and every system for IE.
On the other hand, a variation of the output voltage occurs when the initial load is changed in case the switching frequency is chosen, as fixed value as follows from the intersection point of the dashed trends shown in FIG. 6. Accordingly, this special bias point can be used in a limited way only in view of a preferably constant output voltage. Especially a direct supply of sensitive charges with a narrow value range of the allowed load voltage is impossible when choosing a fixed value of the switching frequency.
If one considers a further important aspect of a system for IE operating at a fixed frequency besides the variation of the output voltage, namely the occurring leakages from FIG. 7, one notices that establishing the operating frequency in the left regime of FIG. 6 is disadvantageous. A variation of the occurring leakages independent of the distance is greatest in the regime of low voltage variations (the coupling independent point).
The level of efficiency of the system for IE accordingly reacts very sensitive to variations of the distance in this regime. Consequently, for systems that make use of the coupling independent point, the positioning is significantly constrained and another aspect of the level of efficiency has to be paid attention that no too significant variations of the distance from the nominal distance are possible.
However, if a switching frequency is chosen in the regime of the steepening to the anti-resonance frequency (for the exemplarily chosen example at 65 up to 70 kHz) a plurality of advantages in view of the trend of the power loss occur. On the one hand, the absolute value of the occurring power loss in this regime is smallest for great distances of the shown example. On the other hand, the variation of the level of efficiency is significantly smaller in this regime. This effect is re-enforced for the efficiency of the partial load once more as can be seen from FIG. 8 at a charging of the source with 20% of the nominal output.
Due to the restriction of the possible switching frequency to the special position of the coupling independent bias point, significant restrictions in view of the performance of the overall system occur which can be circumvented by using the present invention. Exploitation of the coupling independent bias point nevertheless results in an inherent variation of the output voltage. The presence of a certain fluctuation range of the output voltage of the resonant level does not fulfil the requirement for the necessary stability of the output voltage for supplying modern power electrical loads. Thus, exploitation of the “coupling independent point” is only possible for a strongly restricted class of energy consumers on the secondary side. In modern usage scenarios, however, the voltage variation of the supply voltage of those loads becomes inadmissibly great even when using this special bias point. Not least, attention has to be paid to the variation in case of charging applications based on Li-ion technology.